For example, when the air is cold enough, one location may start as snow, then transition to sleet or freezing rain as warmer air above the ground moves in, then change back to snow as the storm’s center of low pressure moves away and deeper, colder air arrives.

A mix of snow, ice and bitter cold is heading for more than 180 million people across a vast stretch of the country.

That level of ice can weigh down trees and power lines, increasing the likelihood of power outages and blocked roads.